First off, make sure you are using the form generator plugin within the subscriber.cgi script, not the admin.cgi script.

I'm going to assume your form is correct since you said it worked before (make sure that if you're using a hidden field for naming the list that the name of the field is "lists" and not just "list": <input type="hidden" name="lists" value="my_list">). If you're getting a blank screen, try uncommenting the "use CGI::Carp" line near the top of subscriber.cgi and see if that doesn't give you a more useful error message.